Inductive power transfer relies on alternating power inductively coupled from conductors and received in a power receiver by inductive transfer from those conductors. The receipt of power may be augmented by field concentrating devices and/or by tuning the power receiver to the power frequency.
This specification relates to systems in which the receiver contains a circuit which is resonant at or near the frequency of the current in the conductors. Known circuits for inductive power transfer may produce substantial reactive power flow when the tuned circuit is detuned to reduce power flow.
The Problem
The problem of unwanted reactive power flow can be almost completely overcome using what are known as “decoupling” controllers, however the choice available to designers is very restrictive. These controllers act by disconnecting the tuned circuit from the load, that is by decoupling the load from the inductive power receiving circuit. The decoupling may occur at a slower rate than the power frequency, or it may occur at a faster rate than the power frequency. In either case the average power taken from the power source is reduced. The relationship between the time coupled and the time decoupled may vary, so that the controller effectively acts to partially variably decouple the load.
If a parallel circuit topology is used for the inductive power receiver, then the maximum load current is fixed and increases in power for short transient situations can only be achieved by increasing the output voltage. Since most equipment is voltage sensitive this is unacceptable. Conversely if the power receiving circuit is a series tuned circuit it has a fixed maximum output voltage but does allow increases in power by transient surge currents. Here the average output voltage may be set by the switch duty cycle but the current may then be allowed to increase transiently as required. However these transient surges correspond to high voltages across the pick-up coil and the tuning capacitor and these may be unacceptable.